The present invention relates to an optical recording medium, and further to an optical recording method.
In optical recording materials, it is well known that a reduction in the wavelength of light to one nth results in an nxc3x97n-fold increase in the recording density of information. Accordingly, the development of short-wave lasers having a wavelength of about 400 nm has been gone ahead with, and the practical use thereof has just started. Conventionally, laser beams having a wavelength of 780 nm to 830 nm have been used, and recording media have also been developed and designed so as to exhibit the optimum performance within this wavelength region. However, with a decrease in the wavelength, light absorption ability, light reflection ability and information capacity required for the media also change.
Optical recording media represented by optical disks are classified into three types: (a) media in which only reading is possible (replay only type), (b) media in which writing is once possible (write once type) and (c) rewritable media (rewrite type), according to the respective characteristics thereof. Of these, the invention relates to the write once-type optical recording media of (b). The write once-type optical recording media are utilized as outboard recorders for electronic computers, and also used for recording music, images and works.
The conventional write once-type optical recording media can be roughly classified into the following five types:
(1) Media in which using thin films of tellurium-containing low-melting alloys, thin films of other metals or alloys thereof, or thin films of organic compounds including cyanine dyes, the recording films are locally evaporated by laser beam irradiation, or the recording films melted are pulled by surface tension, thereby forming pits (pitting type);
(2) Media in which amorphous films such as a tellurium oxide film are locally melted by laser beam irradiation, followed by rapid quenching to induce crystallization, thereby forming pits, and the difference in reflectance between the amorphous and crystalline films is utilized for reading (phase change type);
(3) Media in which metal films each comprising two layers comprising raw materials different from each other are melted by laser beam irradiation to locally alloy them (alloy type);
(4) Media in which bubbles are developed between layers by heat caused by laser beam irradiation to form pits (bubble forming type); and
(5) Media in which fine relief structure surface are formed, which are melt smoothed by heat caused by laser beam irradiation to increase the reflectance (metal oxide semiconductor (MOS) type).
The write once-type optical recording materials of (1) to (5) each have the following problems:
Of the pitting type recording media of (1), ones using the low-melting metals have the problem that errors are liable to occur because edge portions rise to provide the poor shape of pits written, in addition to the problems of keeping quality and toxicity. Further, ones using the organic films tend to deteriorate in their characteristics by ultraviolet rays. The phase change type disks of (2) have small changes in reflectance and are small in the size of signals, so that expensive apparatus are required for reading. For all of the alloy type disks of (3), the bubble forming type disks of (4) and the MOS type disks of (5), the optical recording media have multilayer structures in principle, leading to complicated structures of the disks. Further, many have small changes in reflectance and low writing speed.
Several methods using metals are disclosed, and physical methods such as vapor deposition and spattering and chemical methods such as spin coating of colloidal dispersions of metals are disclosed.
Materials for the recording media in which the thin films are formed by the physical methods using the metals are disclosed as below. Japanese Patent No. 1739116 discloses recording layers in which metals or semi-metals and organic polymers exist as mixtures, and shows a vapor-deposited layer of aluminum and bismuth. JP-A-57-94944 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses thin films of fine particles of metals, alloys, semiconductors or inorganic materials, and exemplifies 20-nm FeNi (9:1), 25-nm Bi, 10-nm Ag, 30-nm Si and 30-nm carbon. JP-A-61-104438 discloses a structure in which fine metal particles are dispersed in organic matrices. In the case of Ag particles, the particle size thereof is 10 nm, and in the case of Au particles, the particle size thereof is 28 nm. JP-A-61-95991 discloses recording layers formed by vapor deposition in which metal or alloy particles having an average particle size of 4 nm to 40 nm are three-dimensionally arranged and the porosity is specified, and exemplifies Ag particles having a particle size of 10 nm and Au particles having a particle size of 15 nm. JP-A62-151394 discloses thin films in which ultrafine particles having a particle size of 100 nm or less are dispersed in organic compounds, and exemplifies titanium black having a particle size of 50 nm. JP-A-63-262286 discloses resin films in which metal particles having a particle size of 10 nm or less are contained, and exemplifies Au particles. JP-A-1-78885 discloses films in which Au and other metals exist as a separated phase without forming alloys. JP-A-7-76171 discloses recording layers comprising Ag or Al particles and alloys containing them, which are discretely distributed and have a size of 100 nm or less, on the condition that lasers having a wavelength of 600 nm or less are used.
The films of the metals or the alloys thereof formed by these physical methods are studied on the condition that the laser beam of 830 nm or 780 nm, which is not a blue laser, is used except for JP-A-7-76171. Accordingly, they are unsuitable for the object of the invention. Although the recording layers described in JP-A-7-76171 are based on the condition that a blue laser is used, the thin films are formed by spattering. It is therefore difficult to control a particle size distribution, and they are not advisable in terms of practical use of light absorption by the original plasmon resonance of metal particles. In fact, in a thin film having a thickness as thin as 1 nm to 2 nm prepared by spattering, an absorption peak was observed in the vicinity of 500 nm, but it was very difficult to obtain a sufficient absorption at less than 500 nm, as shown in FIG. 1. A mere increase in thickness results in exclusively increased absorption in a long-wave region of more than 500 nm. As a result, it was very difficult to improve the recording sensitivity by such physical methods.
Materials for the recording media in which the thin films are formed by the chemical methods using the metals are disclosed as below. JP-A-56-10491 discloses a method of dispersing fine particles of transition metals other than the Group 2B metals or oxides thereof having a particle size of 2 nm to 15 nm in polymers to prepare colloidal dispersions, and forming thin films, and exemplifies Fe and Co particles having a particle size of several nanometers. JP-A-58-53036 discloses a method of dispersing metallic silver by the reducing method or the developing method to prepare colloidal dispersions, and forming thin films. Japanese Patent No. 2686984 discloses a method of utilizing metallic silver-dispersed layers produced by organic silver salt oxidizing agents and reducing agents. JP-A-4-105986 discloses a method of laminating layers containing at least one kind of metal element, wherein one of the metal elements is Pd having a particle size of 5 nm to 50 nm. Of course, these are not studied on the condition that a blue laser is used, so that these are unsuitable for the object of the invention. Further, the methods described in JP-A-58-53036 and Japanese Patent No. 2686984 are complicated in production processes, and therefore, can not be put to practical use.
In view of the above-mentioned circumstances, an object of the invention is to provide a write once-type optical recording medium high in sensitivity and excellent in information keeping quality in a short-wave region in which high recording density can be expected.
Another object of the invention is to provide a recording method thereof.
Other objects and effects of the present invention will become apparent from the following description.
The above-mentioned objects can be attained by an optical recording medium comprising a recording layer containing ultrafine particles of a metal selected from the Group 8 and Group 1B elements, the particles having an average particle size of 1 nm to 50 nm, and surfaces thereof being modified with an adsorptive compound, and by making a recording and replay thereon with a short-wave laser beam having a wavelength of 500 nm or less. Preferred embodiments are described below:
(1) An optical recording medium comprising a recording layer containing ultrafine particles of a metal selected from the Group 8 and Group 1B elements, said particles having an average particle size of 1 nm to 50 nm, and surfaces thereof being modified with an adsorptive compound.
(2) The optical recording medium according to item (1) above, wherein said adsorptive compound is a compound having an xe2x80x94SH group, a xe2x80x94CN group or an xe2x80x94NH2 group.
(3) The optical recording medium according to item (1) or (2) above, wherein said metal is a platinum group element or an alloy thereof.
(4) The optical recording medium according to any one of items (1) to (3) above, wherein each of said ultrafine particles comprises an inner shell and at least one outer shell.
(5) The optical recording medium according to item (4) above, wherein a metal of said outer shell has a nobler potential than a metal of said inner shell.
(6) The optical recording medium according to item (4) or (5) above, wherein said inner shell comprises silver or a silver-containing alloy, and said outer shell comprises a metal having a nobler potential than silver or the alloy containing the metal;
(7) The optical recording medium according to any one of items (1) to (6) above, wherein said ultrafine particles have a monodisperse particle size distribution.
(8) The optical recording medium according to any one of items (1) to (7) above, wherein said recording layer contains a binder.
(9) The optical recording medium according to item (8) above, wherein said recording layer contains a silicone rubber or a polybutadiene rubber.
(10) The optical recording medium according to any one of items (1) to (9) above, wherein the recording layer is formed by applying a colloidal dispersion of said ultrafine particles in a hydrophilic or hydrophobic solvent.
(11) The optical recording medium according to any one of items (1) to (10) above, wherein said recording layer has a thickness of 3 nm to 180 nm.
(12) The optical recording medium according to any one of items (1) to (11) above, further comprising a substrate, a reflective layer and a protective layer, wherein the recording layer, the reflective layer and the protective layer are provided on the substrate in this order.
(13) The optical recording medium according to any one of items (1) to (11) above, further comprising a substrate, a heat insulating layer, a reflective layer and a protective layer, wherein the recording layer, the heat insulating layer, the reflective layer and the protective layer are provided on the substrate in this order.
(14) The optical recording medium according to item (13) above, wherein said heat insulating layer comprises an amorphous fluororesin.
(15) The optical recording medium according to any one of items (1) to (14) above, wherein said recording layer or said heat insulating layer contains an organic dye.
(16) An optical recording method which comprises carrying out recording on an optical recording medium according to any one of items (1) to (15) above using a laser having an oscillating wavelength ranging from 300 nm to 500 nm.